1. Field of the Invention
The invention relates to a doctor blade, in particular for scraping printing ink from a surface of a printing plate, which comprises a flat and elongated base element having a working edge region formed in a longitudinal direction, where the working edge region is coated with at least a first coating based on a nickel-phosphorus alloy. The invention further relates to a process for producing a doctor blade.
2. Description of the Related Art
Doctor blades are used in the printing industry, especially for scraping excess printing ink from the surfaces of printing cylinders or printing rollers. Particularly in the case of gravure printing and flexographic printing, the quality of the doctor blade has a critical influence on the printing result. Unevennesses or irregularities in the working edges of the doctor blade which are in contact with the printing cylinder lead, for example, to incomplete scraping of the printing ink from the ridges of the printing cylinders. This can result in uncontrolled release of printing ink on the printing carrier.
The working edges of the doctor blade are pressed against the surfaces of the printing cylinders or printing rollers during scraping-off and are moved relative to these. The working edges are therefore, particularly in the case of rotary printing presses, subjected to high mechanical stresses which result in corresponding wear. Doctor blades are therefore basically consumables which have to be replaced periodically.
Doctor blades are usually based on a base element of steel having a specially shaped working edge. To improve the life of the doctor blade its working edges can additionally be provided with coatings or surfacings of metals and/or plastics. Metallic coatings often contain nickel or chromium which are optionally present in admixture or alloyed with other atoms and/or compounds. The nature of the coatings in terms of materials has a critical influence on, in particular, the mechanical and tribological properties of the doctor blade.
WO 2003/064157 (Nihon New Chrome Co. Ltd.), which is equivalent to U.S. Patent Publication 2005/0089706, now issued as U.S. Pat. No. 7,152,526, which applications are incorporated herein by reference and made a part hereof, describes, for example, doctor blades for printing technology which have a first layer of chemical nickel containing particles dispersed therein and a second layer having a low surface energy. The second layer preferably comprises a coating of chemical nickel containing fluorine-based resin particles or a purely organic resin.
However, doctor blades which have been coated in this way are still not fully satisfactory in respect of the life and wear resistance. In addition, it has been found that when such doctor blades are used, uncontrolled streaking can occur, especially in the running-in phase, which is likewise undesirable.
There is therefore still a need for an improved doctor blade which, in particular, has both a longer life and also allows optimal scraping-off.